1. Technical Field
The present invention relates to a light-emitting device using light-emitting elements, an electronic apparatus, and a driving method.
2. Related Art
Printers, serving as image forming apparatuses, use a light-emitting device including a plurality of light-emitting elements arranged in an array as a head unit for forming an electrostatic latent image on an image carrier, such as a photorecepter drum. In general, such a head unit has a plurality of light-emitting elements arranged in a line along the main scanning direction. In addition, a light-emitting diode, such as an organic light-emitting diode (hereinafter, referred to as an OLED), has been used as the light-emitting element.
The head unit has light-emitting elements, a driving current source that is provided in the vicinities of the light-emitting elements and supplies driving currents to the light-emitting elements, and a driving circuit that generates a driving signal for controlling the supply of the driving current formed on a substrate. In the line head, when all the light-emitting elements emit light to form a latent image, the driving currents corresponding to the number of light-emitting elements flow at the same time, which causes current consumption to instantaneously increase. When a large amount of current flows instantaneously, a voltage to be applied to the head varies, which may cause an erroneous operation of the head. Therefore, in the line head, it is important to reduce the amount of current instantaneously flowing in order to stably operate the head. In order to achieve this object, the following technique has been proposed: in a light-emitting element array in which a plurality of light-emitting elements are classified into a plurality of blocks and the light-emitting elements belonging to each of the blocks are arranged such that the light-emitting elements are displaced in one of two directions (parallel to a sub-scanning direction) orthogonal to the direction in which the light-emitting elements are arranged, the directions in which adjacent blocks among the plurality of blocks are displaced are opposite to each other (JP-A-2003-80763). In this way, the emission of light alternately occurs in adjacent blocks, which makes it possible to temporally disperse the driving current. As a result, instantaneous current consumption is reduced, which makes it possible to prevent power noise and thus to stably operate the head.
In the related art, light-emitting elements have been arranged in a line, but in the technique disclosed in JP-A-2003-80763, the light-emitting elements are arranged in a plurality of lines, which causes the length of the head unit to increase in the sub-scanning direction. As a result, the structure of the line head becomes complicated, and thus the manufacturing cost and the size of the line head increase. In order to solve these problems, it is preferable to reduce the number of rows of light-emitting elements to two. However, when the number of rows of light-emitting elements decreases, the number of light-emitting elements emitting light at the same time increases, which causes a large amount of current to flow instantaneously, resulting in an increase in noise. That is, the number of rows of light-emitting elements is inversely proportional to the amount of noise generated. In the technique disclosed in JP-A-2003-80763, it is necessary to predetermine the number of rows of light-emitting elements, which makes it difficult to manage when slight variation in the arrangement of the light-emitting elements occurs. In general, it is difficult to predetermine the amount of the instantaneous current required to accurately operate the head (to reduce the amount of noise). Therefore, in many cases, a printing test is performed by actually using a finished head to allow an appropriate measure to reduce noise to be taken. In the technique disclosed in JP-A-2003-80763, if the number of rows of light-emitting elements is increased due to an insufficient measure to reduce noise, a new head unit needs to be manufactured. Therefore, it is necessary to manufacture a head having a small amount of noise, that is, a large number of rows of light-emitting elements in order to reliably operate the head. However, as described above, having a large number of rows of light-emitting elements causes an increase in the manufacturing cost and the size of the head. Therefore, it is difficult to realize a low manufacturing cost, a small size, and a stable operation for the head according to the related art.